Playing video in browser

[excalamus]

I can't figure out how to play the videos on the Guix homepage using a browser 
from within Guix.

I have installed GuixSD in a VM with the XFCE desktop.  I have installed 
Epiphany, along with gstreamer andd ffmpeg.  When I navigate to 
http://guix.gnu.org/videos/, the videos won't play.

I have searched the mailing lists and the Internet in general.  The usual 
answers suggest gstreamer, ffmpeg, or non-free flash plugins for playing HTML5. 
 Clearly, the latter would not be supported by Guix.  

What configuration is required to play videos in the browser?